In general, when a particulate material is compacted in a die, particle-particle friction and die-wall-particle friction result in variations in the applied pressure in the particulate body and, in consequence, the compacted body has a non-uniform pressed density. The non-uniform pressed density of the compact gives rise to a differential shrinkage during subsequent sintering, resulting in distortion of the ceramic article. Even when sintering is not accompanied by shrinkage, the non-uniformity of density remains in the sintered component and is a source of weakness.
Nuclear fuel pellets are ceramic substantially cylindrical solid or hollow bodies which are composed mainly or wholly of an oxide of uranium, especially UO.sub.2. Collections of such pellets are used together in a sheath providing a fuel rod or pin. Assemblies of such rods or pins are employed as the active elements in a nuclear reactor.
In a conventional process for the manufacture of nuclear fuel pellets, a uranic oxide powder, eg manufactured in the manner described in EP 0277708, is compacted or compressed in a die or mould and then sintered usually over a period of several hours at least, usually in a reducing atmosphere at elevated temperatures, eg 1500.degree. C. to 1800.degree. C.
It is desirable for the pellets to be produced with a body shape which is a right circular cylinder. Product specifications normally require such a shape. However, pellets produced in the conventional manner described above may not, after sintering, be obtained with a right circular cylindrical shape even though the cavity of the die or mould used to form the pellet shape is itself a perfect right circular cylinder. The pellets are often produced with a shape which has a body waist in a wheatsheaf-like profile, ie the cross-sectional area (perpendicular to the pellet axis) taken at different points along the length of the pellet is less near the middle of the pellet than near the ends of the pellet. Grinding of the pellets is normally required after sintering to meet product specifications. Such grinding is both time consuming and costly.